Accompanying advancements in technologies, various digital data can be stored in a storage device. People may store or communicate confidential information in the storage device. Once the storage device is lost, the confidential information may be stolen.
Nowadays, the information security is more and more important. The storage device can store an encrypted data instead of the original data for preventing from stealing. If someone wants to take out the original data, it is needed to decrypt the encrypted data according a key. Once a hacker gets the key, he can take out the original data easily. Today, network and smart phone application programs are widely used. If the key is stored in the program or a non-volatile memory, the hacker or researcher can obtain the key. Therefore, how to keep the key in secret is a challenge in the information security.